Aircraft fatigue

The Difference Engine: Old before their time

TAKE a paper-clip and straighten it out. Using just thumbs and fingers, bend it in the middle to form a right-angle. Then, at the same place, bend it back to form a right-angle in the opposite direction. Do that half a dozen times or so and the paper-clip will snap in two. The extraordinary thing about “metal fatigue” is that it takes only a few pounds of force applied repeatedly back and forth across the paper-clip's thickness to break it. To snap a typical paper-clip in tension—by clamping one end and tugging on the other—would require a force of 50lbs or so.

The first to appreciate the catastrophic effects of stress-reversals were railway engineers in the 1840s. Broken axles caused countless accidents as railway lines crept across Europe and America. Being simply a rotating horizontal shaft with a heavy vertical load on it, early locomotive axles suffered severe stress reversals in their outer skins with every rotation. William Rankine, a Scottish engineer and one of the fathers of thermodynamics, was the first to explain how these repeated stress reversals could cause cracks to propagate. By the 1850s, the steam-engine pioneer James Braithwaite had coined the term “metal fatigue”.

The irony is that the lesson had to be relearned a century later. This time it was aircraft manufacturers who suffered the consequences. Their troubles began in the 1950s when they started flying higher and needed to pressurise the cabins of their passenger planes. Two de Havilland Comet aircraft—the world's first commercial jet—broke up mysteriously in mid-air in 1954. Though it all but destroyed de Havilland, the disaster gave the industry crucial insights into how metal fatigue can rip an aircraft suddenly apart. It also taught them how to prevent stresses concentrating at certain points, thereby triggering a fatal tear in the aircraft's skin.

Over the past few weeks, aircraft engineers have found they do not know quite as much about metal fatigue as they thought. The source of the problem that forced the Boeing 737-300 used on the Southwest Airlines flight 812 from Phoenix to Sacramento to make an emergency landing on April 1st, following a five-foot rent appearing in the upper-fuselage skin, has flummoxed engineers and safety officials alike.

By all accounts, it should not have happened. Admittedly, every time an airliner takes off and lands it goes through a demanding cycle of stress reversals. Like the paper-clip, the airframe and its alloy skin are stressed first in one direction as the cabin is pressurised while climbing to its cruising altitude, and then in the opposite direction when depressurised during descent for landing. On average, the short-haul aircraft used by Southwest, a budget carrier based in Dallas, do that half a dozen times a day—year in, year out.

The Boeing 737-300 in question was only 15 years old when its skin peeled open along a riveted lap-joint while flying above 34,000 feet (just over 10,000 metres) with 118 passengers on board. The failure caused the cabin to lose pressure instantly and the oxygen masks to deploy. Within minutes, the pilot had got the plane down to 11,000 feet, where the passengers could begin to breath normally again. Shortly thereafter, the plane landed at a military base without further mishap or serious injury.

Much has been made of the 737-300's age. But a commercial aircraft that is 15 years old is still in its prime of life. The real issue is the way Southwest works its fleet so aggressively, specialising in rapid turnarounds. As a result, the plane concerned had accumulated nearly 40,000 flight cycles. An aircraft of that type and age would normally be expected to have logged little more than 30,000 flights.

Over the years, Boeing has probably accumulated more data on the fatigue life of airframes than any other plane-maker. After modifying the lap-joints in the roof following some early failures, the company felt confident that its older 737s would be good for 60,000 cycles before they needed to be thoroughly tested for hairline cracks that could lead to fatigue failures. As an emergency precaution, the Federal Aviation Administration has now said that carriers operating 737s with the same lap-joint design along the roof should inspect the planes after no more than 30,000 cycles. Planes that have already logged 35,000 flights or more have to be inspected immediately.

What makes aircraft fatigue such an dark art is that, unlike standard tests done in a laboratory, an aircraft's structure has to endure a complex, mostly random, set of static as well as cyclical stresses when in service. Impurities in its material affect the fatigue life. So does the material's hardness, and especially its surface condition. How the components were heat-treated in the factory is another factor. The operating temperature makes a difference, too. Worse still is the structural component's shape: notches and sharp corners create concentrations of stress that can initiate cracks. The square windows on the original Comet jetliner were found to be the primary cause of its disintegration. Airliners have had windows with rounded corners ever since.

All things being equal, which they rarely are, the higher the cyclical stress level on an aircraft structure, the fewer the number of reversals it can withstand before breaking. As the stress level is gradually reduced, there comes a point where a structure can survive enough stress reversals to exceed the component's expected life. By convention, the stress level that allows a component to survive 10m reversals is called its “endurance limit”. Unfortunately, the endurance limit is not some absolute—nor even repeatable—value. When tested, identical samples can give widely different results.

In an ideal world, planes would be made of steel. Regrettably, that metal is too heavy for the job. But steel alloys subjected to cyclic stress levels below their endurance limit rarely fail as a result of fatigue. In other words, they can be made to have an infinite life. The aluminium alloys that are used for their strength and lightness in aircraft construction are just the opposite. None can live indefinitely, and all will fail sooner or later from fatigue. Why that is so lies buried in the different crystalline structures of the two materials. And that is something aircraft designers have to live with.

So what is an aircraft engineer to do? First, perform thousands of fatigue tests in the laboratory and then take a probabilistic view of things. Second, adjust the statistical results downward to account for differences between test conditions and the real world. Third, factor in all the known statistical variations of the material itself. The aim, as always, is to ensure that unpredictable factors do not reduce the fatigue life of a structure to less than that required.

Maddeningly, as Boeing has found of late, that is easier said than done. The biggest problem aircraft engineers face is that the fatigue cycles a component faces are cumulative. No amount of resting can reset the clock. While an engineering student, your correspondent spent one summer crack-testing engine parts in an airline's workshops in Spain. Parts that passed the stringent inspection were certified accordingly—and stored ready for reuse. What neither he, nor anyone else, could possibly know was whether the part in question would fail in the next 1,000 reversals or last for 10,000 more. No matter what Boeing does to rectify its aging 737s, their airframes are clocking up stress reversals that can never be expunged.

To be pedantic the paper-clip is a poor analogy because it is being bent beyond its elastic limit (so it stays bent until you bend it back). If you stay inside this limit (so it springs back) it will not fail - it is steel after all. The problem is that aircraft alloys do still fatigue even if only bent elastically (and temperature cycled...) - this was unexpected (not forgotten). And of course you can reset the 'fatigue clock' by annealing at a high temperature - but this cannot be done for an aircraft.

The aircraft structure failed, and it failed before its time, but it failed in a relatively benign way. In contrast to the DevHavilland Comet, the crack was stopped by rivets in the right place. It has been known since the failures of the Liberty Ships (welded WWII steel ships that tended to split right in two) that rivets can function as crack arrestors.

This failure, while alarming, caused no fatalities or injuries and shows the efforts and lessons learned by engineers over the last two hundred years.

To echo what Philip OCarroll said: to have this plane fail in this way above 30,000 feet (meaning explosive decompression when it happened) and still have a flyable plane is really remarkable. (By "flyable", I mean that it didn't tear itself apart in midair - it could be flown to landing. I do not mean that it can take off again.)

First, it has been reported that "[a]t an altitude above 34,000 feet (10,360 meters), . . . pilots would have had only 10 to 20 seconds of 'useful consciousness' to get their oxygen masks on or pass out." This is unsettling to say the least. Imagine if this happened when one of the pilots was on a break.

Second, as more and more composite materials are used in the commercial aircraft of the future (e.g., the Boeing 787 Dreamliner), no one knows exactly the ability of such materials to withstand constant stress stemming from pressurization changes and other factors. Yes, the United States has had experience with military aircraft in this regard, such as its stealth aircraft. However, query whether the military works its fleet as aggressively as Southwest, which specializes in rapid turnarounds?

A bent and re-bent paper-clip fails because of work-hardening, a quite different phenomenon from, and having little to do with, metal fatigue. And because of its small diameter, the few pounds needed to bend the wire set up stresses in it that are quite as great as those due to the several tens of pounds needed to stretch it in straight tension. Babbage needs a little technical education.

Most of the physical problems which bedevil humanity occur primarily at the microscopic level, catastrophic structural failure among them.

Until we assemble the existing technological pieces using step miniaturization techniques so as to allow ourselves to "go to" Microscopia in captured body movements, vision and hearing, we will be at their mercy.

Specifically, aircraft would have thousands of microscopic inspection stations salted throughout their structures which would allow on site inspection at the most discrete level, even during mid-flight, so technicians could see the beginnings of structural failure long before they became catastrophic.

15 Years OK. How many hours? There are B52 bombers which have been flying for over 50 years. For much of their life, they flew very little, being the US frontline nuclear deterrent response aircraft.
Planes that fly passengers (especially short-haul) these days are often turned around in half an hour or less.

I am surprised that none of the articles and posts discussing the recent stress crack failure on Southwest flight 812 have mentioned the much more spectacular similar failure of a 737-200 flying as Aloha Airlines flight 243 in 1988. A large section of the top of the fuselage blew out, and a flight attendant died. The amazing thing, to anyone who saw a photo of the plane later (see https://secure.wikimedia.org/wikipedia/en/wiki/Aloha_Airlines_Flight_243), was that the plane was able to make a successful emergency landing. The failure was blamed on metal fatigue exacerbated by corrosion due to the coastal environment of the aircraft's operation.

Does this mean that the "Precautionary Principal" now takes precedence? Having lived through the Comet disasters, it seems doubt now means turn-around and flying hours are, in terms of possible failure, questionable in measuring fatigue failure.

So, now that we have identified the problem, we just have to wonder whether the incompetent bureaucrats at the FAA will give extensions and waivers to airlines to skip inspections for an extra few years, as they did to Alaska Air for its famous Flight 261.

Whereas the number of compression-decompression cycles for an individual aircraft is the only reported measure in the metal fatigue
dilemma, I propose what may well be is a more significant or related measure. I propose that dwell-time be measured, the period of time between the noted cycles. My working hypotheses is: the shorter the cumulative dwell-time for any individual aircraft, the greater the prevalence of metal fatigue. The data for analysis are readily available for each aircraft, held in the existing records for each aircraft. I am neither a metallurgist or related, but have formal training in epidemiology. Recollect that Louis Pasteur was a chemist, and his contribution to mankind was monumental. And that Alexander Fleming of penicillin fame was a Fellow of the Royal College of Surgeons. So, why not begin a tentatative analysis of the data, sitting there awaiting analysis. SAILRIPPER

As a child my father and I watched the movie "No Highway in the Sky" starring Jimmy Stewart as an aeronautical engineer convinced that after a certain amount of flight hours a certain model of plane with fail in a specific manner. The movie, being over 50 years old, brings up a point that even with all of our modern technology there is still much to be learned. Watch the movie, if you can find it, it is a classic that should be remembered.

Unless I missed it, neither the article nor the comments to this point have mentioned composites. Though I'm not an engineer, I understand that these will be used extensively in the Boeing Dreamliner and probably other aircraft as well. These have different fatigue characteristics than metal. Will they offer advantages, or do they also introduce uncertainties which make their ultimate success unclear?

I acknowledge that regardless of the future of composites, we will be dealing with metal fatigue issues in the fleet for a long time to come.

@Penhdragon
If you are referring to the B747-SR JAL flight 123 that crashed in Japan in August 1985, yes, it was running a Tokyo-Osaka shuttle with 524 passengers and crew (of which 4 were miraculousy found alive the day after the crash). However, I don't believe Boeing's official line about failure of the rear bulkhead, and suspect an APU explosion as the actual cause of the crash.